[PATCH -tip/core/rcu] Fix rcu_migrate_callback() to allow for multiple waiters

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

The current interaction between rcu_barrier() and CPU hotplug can result
in multiple tasks waiting on rcu_migrate_wq: there can be one CPU-hotplug
notifier, and, because rcu_barrier() invokes wait_migrated_callbacks()
outside of rcu_barrier_mutex, an arbitrarily large number of tasks
executing in _rcu_barrier().  This situation could result in hangs,
because rcu_migrate_callback() would wake up but one task.

This patch addresses this problem by awakening all sleepers.

Located-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---

 rcupdate.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
index bd5d5c8..6c59e1b 100644
--- a/kernel/rcupdate.c
+++ b/kernel/rcupdate.c
@@ -211,10 +211,17 @@ void rcu_barrier_sched(void)
 }
 EXPORT_SYMBOL_GPL(rcu_barrier_sched);
 
+/*
+ * Wake up anyone waiting on migration of RCU callbacks from a CPU
+ * going offline.  There can be only one CPU-hotplug notifier waiting,
+ * but there can be any number of rcu_barrier() invocations waiting,
+ * due to the fact that rcu_barrier() does its wait outside of the
+ * rcu_barrier_mutex.
+ */
 static void rcu_migrate_callback(struct rcu_head *notused)
 {
 	if (atomic_dec_and_test(&rcu_migrate_type_count))
-		wake_up(&rcu_migrate_wq);
+		wake_up_all(&rcu_migrate_wq);
 }
 
 extern int rcu_cpu_notify(struct notifier_block *self,